1. Field of the Invention
The present invention relates to a thermosensitive recording material. More particularly, the present invention relates to a thermosensitive recording material free from a white powder-forming phenomenon on colored images, having a high whiteness, and capable of recording thereon colored images having an excellent persistency.
2. Description of the Related Arts
It is known that a conventional thermosensitive recording material comprises a supporting substrate, for example, a paper sheet, synthetic paper sheet, or plastic resin film, and a thermosensitive colored image-forming layer formed on a surface of the supporting substrate and comprising an electron-donative dye precursor, for example, a leuco basic dye, an electron-acceptive color-developing agent consisting of an organic acid substance, for example, a phenol compound, and a binder. When the thermosensitive colored image-forming layer is heated, colored images are recorded thereon by a reaction of the dye precursor with the color-developing agent.
This type of thermosensitive recording material is disclosed in Japanese Examined Patent Publication Nos. 43-4,160, 45-14,039 and 48-27,736, and is widely employed in practice.
Namely, the thermosensitive recording material is advantageous in that colored images can be easily formed only by heating, and the recording apparatus can be made relatively compact and small size, at a low cost, and is easily maintained, and thus is useful as an information-recording material for various outputs or printers used with, for example, computers, thermosensitive facsimile machines, automatic ticket-vending machines, scientific measurement recorders, and CRT medical measurement recorders.
Current information-recording machines can be operated at a very high speed not possible in the past, and in line with this increase in the operating speed of the information-recording machines, the thermosensitive recording material now must have a significantly enhanced thermosensitivity.
Many attempts have been made to meet the requirement of an enhanced thermosensitivity of the recording material, and among these attempts, important is a development of a new type of color-developing agent, able to replace a conventional color-developing agent, consisting of 4,4'-isopropylidenediphenol (referred to as BPA hereinafter). BPA is advantageous in that the price is low and colored images having a relatively high persistency can be formed thereon.
Nevertheless, BPA is disadvantageous in it has an unsatisfactory thermal sensitivity, due to a relatively high melting temperature of 156.degree. C. thereof.
Many attempts have been made to provide a new type of color-developing compound usable in place of BPA, and some of those attempts are disclosed in Paper and Pulp Technology Times, Vol. 30, No. 5 (1987) to Vol. 31, No. 2 (1988). The disclosures in the above-mentioned publication, led to the development of several hundred types of compounds for BPA.
Among the new compounds, Japanese Unexamined Patent Publication (JP-A) No. 52-140,483 discloses benzyl p-hydroxybenzoate, which exhibits a high sensitivity and a high suitability for a high speed recording machine.
Although benzyl p-hydroxybenzoate effectively causes the resultant recording material to exhibit a high suitability for a high speed recording machine, the resultant colored images developed thereon are disadvantageous in that, when stored for a long time, an undesirable white powder is formed on the colored image surfaces. This phenomenon is referred to hereinafter as a white powder-forming phenomenon.
Also, the colored images exhibit an unsatisfactory resistance to oily or fatty substances and plasticizers, and therefore, if an oily or fatty substance or plasticizer-containing resin article is brought into contact with the colored images, the images fade and ultimately disappear.
Various attempts have been made to eliminate the above-mentioned disadvantages of the conventional thermosensitive recording material, by adding a phenolic antioxidant to the thermosensitive colored-image forming layer or by forming a protective surface layer on the thermo-sensitive colored image-forming layer.
For example, Japanese Unexamined Patent Publication Nos. 60-78,782, 59-167,292, 59-114,096 and 59-93,387 disclose a thermosensitive colored image-forming layer containing a phenolic antioxidant.
Japanese Unexamined Patent Publication No. 56-146,796 discloses a protective layer formed from a hydrophobic polymeric compound emulsion on a thermosensitive colored image-forming layer.
Japanese Unexamined Patent Publication No. 58-199,189 discloses an intermediate layer formed from a water-soluble polymeric compound solution or a hydrophobic polymeric compound emulsion on a thermosensitive colored image-forming layer, and a protective surface layer formed by applying an oil paint containing, as a resinous component, a hydrophobic polymeric material on the intermediate layer.
In the thermosensitive colored image-forming layer containing, as a color-developing agent, benzyl p-hydroxybenzoate, the addition of the phenolic antioxidant, which effectively enhances a resistance of the resultant colored images to oily or fatty substance or to a plasticizer, is disadvantageous in that the resultant thermosensitive colored image-forming layer is easily stained in the ground thereof, and thus exhibits a lowered whiteness, and when stored at a relatively high temperature, for example, about 60.degree. C., non-color-developed portions of the colored image-forming layer exhibit an undesirably increased color density.
Also, the improvement in the resistance of the resultant colored images to oily or fatty substances or a plasticizer, due to the phenolic antioxidant, is still not satisfactory.
Furthermore, the phenolic antioxidant does not satisfactorily inhibit the white power-forming phenomenon on the resultant colored image surface.
Also, the arrangement of the protective layer on the colored image-forming layer does not result in a satisfactory persistency of the resultant colored images under high temperature-high humidity conditions.
Further, when a recording material having a protective layer formed on a colored image-forming layer is subjected to a cutting procedure or an adhering procedure, an oily or fatty substance penetrates the colored image-forming layer through the cut edges thereof, and thus the resultant colored images fade and ultimately disappear. Therefore, the protective layer cannot completely eliminate the disadvantages of the conventional recording sheet.